I have done exactly that before and it works very well, but it does have drawbacks. One drawback that comes to mind is if the load (antenna) has a very low Zin. This will cause the output transistor to conduct much higher current than with a matched load and could cause failure of the PA transistor, without the protection ever kicking in.
In my application I was using the sampled RF output for ALC and the SWR protection came as a bonus. However, I suspect a directional coupler will provide better control and they're not difficult to make.
Dick